In many situations there is a desire to produce a stream of pressurized fluid at a flow rate that is below the minimum or output rate of a pump that comprises part of a particular piece of equipment or that is situated at a particular site or location. For example, in many oil or gas field applications (including underbalanced drilling situations) it is necessary to pump or inject fluid into a wellbore at rates below the output rate of the pump that is supplied with a drilling or service rig. In such cases a separate pump that is designed to pump at the low rate required for a specific operation must typically be acquired and put to use.
If properly sized for a given application, utilization of a secondary pump will satisfy the particular needs at hand, but also presents a number of limitations. First, a secondary pump and pumping system can add significantly to cost in terms of capital outlay or rental charges, and also in terms of increased operating and maintenance costs. In addition, a secondary pump designed for producing a stream of pressurized fluid at a particular flow rate will, as in the case of the primary rig pump, be of little use in the event that the operational parameters should change and should there be a need or desire for a fluid flow rate below the minimum pumping rate of the secondary pump. Where there is the expectation that a required minimum flow rate may change, more complex and sophisticated pumps having variable flow rates can be utilized, but once again only at significantly increased costs.